i have a gpt partition table with a lvm volume on it and when i boot, the lvm vg does not seem to be activated(cannot find root device or something)
if i go to shell i can (only control in /dev/mapper/)

I use an initrd which I generated ages ago with the older genkernel script.

Please notice, my personal expierence: You just need an initrd for booting your kernel and getting your rootfs enabled and loaded. It does not matter how old the initrd is as long it helps the kernel to see the rootfs and loads the init process.

As my setup is much more complicated I can only point you out to some keywords, which may help you: initrd + busybox + grub2 + genkernel (these keywords may help you to solve your problem)

Please also notice that anything important should be build in your kernel and your kernel and bootloader and initrd must reside outside of lvm in a bootable partition

EDIT: For me it seems you have an issue with your bootloader, assuming grub2m first.

Well if you could provide your relevant section of your bootloader, e.g. hole passage with the loading / initrd ... it may helps. It may also help when you post exactly the genkernel command parameters and version you used, It may be also useful if you could pastebin your kernel config file too.